Referring to FIG. 1, a typical Doherty type amplifier known to the prior art includes a primary power amplifier 14 and an auxiliary power amplifier 18 whose input terminals 16 and 20 respectively are connected together at node 22. The input terminal 20 of auxiliary amplifier 18 is connected to node 22 through a phase shifter 24. Node 22 is the input terminal for an input signal 23 such as an RF signal. The output terminals 26, 28 of amplifiers 14 and 18 respectively are connected together at node 32 which is the output terminal for the amplifier pair. The output terminal 26 of primary amplifier 14 is connected to node 32 through an impedance inverter 30. Output node 32 provides the amplified output signal to a load 34.
In operation, the input signal 23 is amplified by primary amplifier 14 and passed through the impedance inverter 30 prior to being transmitted to load 34. The auxiliary amplifier 18 is turned off at this point. As the voltage applied by the primary amplifier 14 increases, the auxiliary amplifier 18 turns on. Typically the auxiliary amplifier 18 is a class C amplifier.
FIG. 2 depicts a power graph for the Doherty amplifier shown in FIG. 1. As the voltage supplied by the primary amplifier 14 increases, the output voltage increases linearly until the primary amplifier 14 reaches its output limit, 36. Eventually, the primary amplifier 14 reaches saturation, and its output voltage approaches its saturated limit. When a saturation point 37 is reached by the primary amplifier 14, the auxiliary amplifier 18 is turned on and as the input voltage is increased the output voltage is also increased linearly. The transition period marked by the powering on of auxiliary amplifier 18 is typically non-linear. Thus, a need exists for a Doherty style amplifier capable of amplifying signals without the non-linearities introduced by the powering on of the auxiliary amplifier.